The renal medulla is intimately involved in the genesis of hypertension; prostaglandins (PG), some of which are produced in the renal interstitial cells, have a well-known antihypertensive effect. Since an increase in the salt concentration changes the number of osmophilic granule-like droplets in renal interstitial cells, the relationship between PG metabolism, droplet formation, H20 transport, ADH action, and the osmotic condition of the renal papilla was investigated in both in vitro and in vivo situations. In nearly all cases, we found evidence that higher urea concentrations have substantial effects of PG metabolism in the rabbit kidney, including: an apparent increase in osmophilic granules, an increase in arachadonic acid, a PG precursor, and a concurrent decrease in malonyldialdehyde formation. We are now progressing along several lines of investigation. The main thrust is to measure PG release and tissue content (papillary) under various conditions of osmolality (Urea, NaCl, mannitol). We hope to establish an in vivo and in vitro relationship between hyperosmolality, especially urea-induced and PG metabolism. BIBLIOGRAPHIC REFERENCES: Van Praag, D. and Farber, S.J. Effect of increased osmolality on prostaglandin metabolism in rabbit kidney. Fed. Proc., 35: No. 3, March 1, 1976. Manger, W.M., Van Praag, D., Weiss, R.J., Hart, C.J., Hulse, M., Rock, T.W. and Farber, S.J. Effect of transplanting renomedullary tissue into spontaneously hypertensive rats (SHR). Fed. Proc., 35: No. 3, March 1, 1976, p. 556, #1931.